The present invention is directed to label and tape constructions that exhibit clean removability from various substrates.
Pressure-sensitive adhesive labels and tapes are well known. In a typical label construction, one or more layers of adhesive are coated on or otherwise applied to a release liner and then laminated to a backing, such as paper, polymeric film, or other ink-receptive, flexible material. In a typical tape construction, one surface of a polymeric film or woven paper is coated with an adhesive, and the construction is then wound up upon itself. A release liner is not generally required. Labels are usually die-cut and matrix-stripped before use. In contrast, tapes usually do not require die-cutting and matrix-stripping, and generally need not be ink-receptive.
Pressure-sensitive adhesives (PSAs) must be formulated to fit specific performance requirements, including sufficient shear, peel adhesion, and tack or quickstick, at various temperatures and on a variety of substrates. PSAs can exhibit a range of properties and are used in a broad spectrum of applications. Depending on the choice of facestock to which they are laminated, and the substrate(s) to which they are applied, PSAs may be classified as more or less xe2x80x9cpermanent,xe2x80x9d xe2x80x9cremovable,xe2x80x9d or xe2x80x9crepositionable.xe2x80x9d When a permanent PSA tape or label is adhered to a substrate, the adhesive bond to the substrate grows over time, and the backing material cannot be removed without damaging the backing and/or the substrate, or without leaving an adhesive residue on the substrate. In contrast, removable PSAs can be removed from a substrate by application of a relatively small peel force, even after an extended period of time, because adhesion to the substrate remains constant, or grows only slightly over time.
At least two approaches to achieving good removability have been tried: control of the adhesive coating process and control of adhesive rheology.
It is known that adhesive peel forces can be reduced by minimizing the coat weight of the adhesive in the label or tape construction. Even at low coat weights, however, adhesion generally will grow with age, particularly on plastic and weak paper facestocks, and the adhesive will become non-removable due to the inherent permanent characteristic of most adhesives. One alternative to reducing the coat weight of the applied adhesive composition in a PSA construction is to use a discontinuous coat of adhesive segments over portions of the label or tape construction. This is described in European Patent 0180598. Another alternative is to include silica microspheres in the adhesive composition. This reduces the adhesive content of the applied adhesive composition. Examples of this approach are found in U.S. Pat. No. 3,691,140 (Silver), U.S. Pat. No. 4,166,152 (Baker et al), U.S. Pat. Nos. 4,495,318 and 4,598,212 (both to Howard), and U.S. Pat. No. 4,810,763 (Mallya et al.)
A rheological approach to rendering an adhesive removable is to design a high gel, low glass transition temperature (Tg) adhesive having a low tangent delta (tan xcex4) at the de-bonding frequency. Adhesives having such properties behave like soft materials and exhibit a low adhesion force during the peel test. However, such adhesives can exhibit poor converting performance, including poor guillotining, due to adhesion between the adhesives and cutting blades and knives.
In order to be employed in modern label manufacturing apparatus, adhesives used in PSA labels must have good converting properties. During label manufacture, the label construction (i.e., a laminate of a face stock, PSA layer(s) and a release liner) is passed through apparatus that converts the construction into label stock and finished products, i.e., sheets or rolls of labels. The processes involved in the converting operation include slitting, die-cutting, and matrix-stripping to leave labels on a release liner; butt-cutting of labels to the release liner; marginal hole punching, perforating, fanfolding, guillotining, and the like. Die-cutting involves cutting of the laminate to the surface of the release liner. Hole punching, perforating and guillotining include cutting clean through the label/release liner laminate.
Unfortunately, removable and repositionable adhesives often exhibit poor converting properties. The viscoelastic nature of such adhesives can hamper precise and clean penetration of the die-cutting blades and slitting knives, and disrupt guillotining, die-cutting, and other converting operations. This can have commercially unacceptable consequences, as the cost of converting an adhesive laminate into a finished product is a function of the speed and efficiency at which the various processing operations occur. The choice of face stock in the label construction can also impact the ease and cost of convertibility, as well as label removability and repositionability.
Efforts to formulate removable and repositionable adhesives that convert well have met with mixed results. U.S. Pat. No. 4,548,845 (Parsons et al.) describes the use of polyalkylene glycols, such as polyethylene glycol (PEG), with tackified water insoluble elastomers to reduce the bonding between an adhesive and knife during the guillotining process. Unfortunately, phase separation and incompatibility between the adhesive system and PEG appear to have limited its amount and usage. U.S. Pat. No. 4,151,319 (Sackoff) and U.S. Pat. No. 4,346,189 (Laurent) describe the use of silicones to reduce the bonding between adhesives and cutting knives. The products display good guillotinability and initial repositionability. Unfortunately, repositionability is lost with time. International Patent Application Publication No. WO 93/10177, incorporated by reference herein, describes removable, repositionable and guillotinable PSA constructions made with a backing coated with an acrylic emulsion PSA modified by a wax, surfactant, or mixtures thereof. In some embodiments, a plasticizer is included. Although the invention described in the international application represents a positive advance in the art, a need exists for improved, removable, emulsion acrylic PSAs and PSA constructions, particularly for use with plastic and weak paper facestocks, where the constructions may be (repeatedly) removed and repositioned.
The present invention provides improved, removable, PSA compositions and constructions, including constructions made with plastic or weak paper facestocks, which heretofore have been more difficult to use in removable and repositionable label applications. In one embodiment, a PSA composition contains an inherently tacky, emulsion acrylic PSA copolymer compounded with an emulsifiable wax and a plasticizer. Removability of the composition is significantly improved by including in the copolymer a small amountxe2x80x94up to about 1% by weightxe2x80x94of a multifunctional crosslinking monomer. Removal of label or tape constructions prepared in accordance with the invention requires a substantially smaller 90xc2x0 peel force, from a variety of substrates, than that required to remove a construction made with a comparable PSA composition differing only in its absence of a multifunctional crosslinking monomer. The peel force builds with time to far less extent than that seen with comparable constructions that lack the multifunctional monomer.